Radiotherapy involves the application of ionizing radiation to a target within a patient (e.g. a tumour) so as to damage the unhealthy cells within the target, eventually causing cell death through one or multiple exposures. The radiation is harmful to both the unhealthy tissue within the target and the healthy tissue which surrounds it, and thus much research has been focussed on maximizing the radiation dose within the target while minimizing the dose outside the target.
Prior to beginning a course of radiotherapy, volumetric images of the patient, and specifically the target region, need to be obtained so that a plan for the treatment can be constructed. The aim of the treatment plan is to establish how to apply the radiotherapy to the patient so that the target region receives the desired dose, whilst the surrounding healthy tissue receives as little dose as possible.
Radiation is delivered in one or more beams of high-energy radiation directed towards a patient. The patient typically lies on a couch or patient support, and the beam is directed toward the patient from an offset location. During treatment, the beam source is rotated around the patient while keeping the beam directed toward the target point. The result is that the target remains in the beam at all times, but areas immediately around the target are irradiated only briefly by the beam during part of its rotation. In this way, the dose to the tumour is maximised whilst the dose to surrounding healthy tissue is reduced.
The cross-section of the beam can be varied by way of a range of types of collimator, such as the so-called “multi-leaf collimator” (MLC) illustrated in EP 0,314,314. The MLC comprises one or more banks of thin, elongate leaves, each movable across the radiation beam to a greater or lesser extent in order to block the radiation. Collectively, the bank of leaves defines a shaped profile which is imparted to the radiation beam. These can be adjusted during treatment so as to create a beam whose cross-section varies dynamically as it rotates around the patient.
Other aspects of the radiotherapy apparatus can also be varied during treatment, such as the speed of rotation of the source and the dose rate. The patient table supporting the patient during therapy may allow movement of the patient during therapy, so as to place the target at an optimum position for treatment. There are a large number of variables offered by the apparatus in order to tailor the radiation dose that is delivered to the patient.
The volumetric images are therefore analysed to identify a target region into which a minimum dose is to be delivered, any sensitive regions such as functional organs for which a maximum dose must be observed, and other non-target regions into which the dose is to be generally minimised. This three-dimensional map must then be used to develop a treatment plan, i.e. a sequence of source movements, collimator movements, and dose rates which result in a three-dimensional dose distribution that (a) meets the requirements as to maximum and minimum doses (etc) and (b) is physically possible, e.g. does not require the source to rotate around the patient faster than it is physically capable.
The treatment plan is therefore a set of instructions to be carried out by the various components of the radiotherapy system in order to have a particular therapeutic effect in the patient.
Throughout the treatment process, safety of the patient is paramount. To this end, prior to treatment, each treatment plan is carefully reviewed, with complex plans being tested first on a phantom to check the radiation dose distribution is as expected. If an unexpected or otherwise unacceptable radiation dose results from the treatment plan, a revised treatment plan can be generated.
Further, each component of the radiotherapy system is monitored prior to and during treatment to ensure it is working correctly. If a component develops a fault, the treatment can be suspended immediately to reduce the potential for harm to the patient.
The nature of such monitoring depends on the particular component. However, as an example, it is known to place a marker at the tip of each MLC leaf and to monitor the leaf positions using a camera. By comparing the actual leaf positions with the expected leaf positions, the functioning of the MLC leaves can be tracked.